Back in December, the “4.15 Strong” group started its first training run at the Boston Marathon finish line. The location made perfect sense, giving the group’s many first-time marathon runners easy access to loops of different distances along the Charles River.

But the run, launched from the site of the first bomb explosion, served a more essential purpose: It addressed the toughest mental and emotional challenge the 4.15 Strong team will face Monday.

All the runners in the group are bombing survivors. For them, the finish line looms larger because of what they experienced last year.

“The mental challenges are as big as the physical ones because I know mentally what some of the folks are dealing with,” said Dave Fortier, who organized group runs and other activities for 4.15 Strong.

“It gives you pause because you’re remembering exactly what happened. For a lot of people, it was their first day back there. For me, it was, ‘OK, let’s get the elephant out of the room quickly. Let’s just get back onto Boylston Street.’

“I wanted them to deal with it early rather than it become a big issue.”

Moments before the first bomb went off, Fortier was running down the left side of Boylston Street, high-fiving spectators and looking at the finish line of his first marathon. He saw that the left side of the line was a bit crowded, so he started to drift toward the right.

‘It’s almost like claiming your stake again, saying, “This my territory.” Crossing that finish line is going to be closing that chapter of the book and saying, “All right, that’s done with. Let’s continue to move on.’’ ’ - Lee Ann Yanni

Then he saw a large flash to his left and felt a radiating warmth. He ended up in a heap on Boylston Street. A piece of shrapnel cut through the top of his right shoe and into his foot. There was a ringing in his ears that remains to this day.

Finishing what they’ve started is big for the rest of the 4.15 Strong runners, too.

Fortier affectionately describes 4.15 Strong as “a ragtag group,” one that came together after the BAA gave survivors two entries each to the 2014 Marathon. Many survivors gave the numbers to friends and family, but about 30 decided they would attempt to go the distance themselves. Most for the first time.

From his experience running Boston and New York (last November), Fortier knew there was a big leap from deciding to run a marathon to preparing the right way. So, along with fellow survivor and marathon runner Lee Ann Yanni, Fortier figured out what kind of support 4.15 Strong would need, organizing long runs and informational clinics, and recruiting 1976 Boston winner Jack Fultz as team coach.

“As runners, we’re one gigantic family,” said Yanni, of Boston. “Then we have an even tighter family that is essentially running for the same reason: to prove that we’re stronger now than we were before.”

Waiting at the finish line for one of her physical therapy patients to run by, Yanni felt something warm brush against her leg when the first bomb exploded. She looked down and saw bone sticking out and blood gushing. She hopped into Marathon Sports, where her husband grabbed T-shirts and shorts to try to stop the bleeding.

While Yanni was being rushed to Tufts Medical Center for emergency surgery, she thought about the Chicago Marathon she had registered to run in October.

She still planned to complete that race — and, on roughly five weeks of training, she did.

“I was like, ‘It’s just my fibula,’ ” said Yanni. “ ‘They’re going to put plates and pins in. I should be able to get back in such-and-such a time. Chicago is six months away. I’m going to make it happen.’ ”

But she really wanted to run Boston. Over the winter and spring, Yanni could be found training with Fortier and the rest of 4.15 Strong team.

They started some runs at the Marathon Sports store in Brookline, tackling the Newton hills.

The group can be hard to miss in their bright yellow, long-sleeved T-shirts that read “4.15 Strong” on the front and “Survivor” on the back. “It’s almost like claiming your stake again, saying, ‘This my territory,’ ” said Yanni. “Crossing that finish line is going to be closing that chapter of the book and saying, ‘All right, that’s done with. Let’s continue to move on.’ ”